1. Field of the Invention
The invention relates to superconductive devices, and in particular to a method of fabrication of niobium tunnel junction devices.
2. Description of the Prior Art
Superconductive Josephson memory and logic circuits are known in the art which utilize the Josephson tunnel junction as the active switching elements or gates therefor. The Josephson junction comprises two superimposed layers of superconductive material with an insulator or semiconductor layer or barrier therebetween whereby Josephson tunneling current flows from one superconductive layer to the other through the barrier via the Josephson tunneling effect. With the superconductive layers connected into a superconductive loop and control lines disposed adjacent the junction, the d.c. Josephson zero voltage current flowing through the device may be controlled as to provide the necessary current steering control functions in the Josephson circuitry.
Nb-Insulator-Nb tunnel junctions offer distinct advantages, in both performance and reliability, over similar junctions made with other superconducting materials (such as Pb and Pb alloys). Nb is stronger, has a higher superconducting transition temperature and is more resistant to degradation from thermal cycling. Furthermore, an oxide of Nb (Nb.sub.2 O.sub.5) is just an excellent dielectric that is suitable as the tunneling insulator in Josephson tunnel junctions.
Sequences used to date to fabricate Nb tunnel junctions require fabrication steps where sensitive device surfaces are exposed to undesirable contaminants. The tunneling oxide layers must be formed on a surface that has been exposed to the atmosphere, covered with photoresist materials and been subjected to various chemical plasma or sputtering cleaning procedures. In most cases, the ultra-thin oxide (&lt;50 angstroms) itself is exposed to and contaminated by the atmosphere between its formation and the Nb counter electrode deposition. In any case, controlled Nb-Insulator-Nb devices have not been achievable using prior art fabrication techniques.